This invention relates to an antenna assembly mountable onto a housing for a radio device. The retractable antenna assembly may be applied, for example, to a portable radio and, in particular, to a hand portable radio telephone, such as a cellular telephone.
A radio intended for two-way communication generally operates with either an external rod antenna or with an internal antenna. An external rod antenna may be of the fixed type, having a predetermined length, or of the retractable type. Subject to the operating transmission frequency, fixed length antennas may be relatively short, however, they are not conducive to compact design nor are they particularly suitable for a radio intended to be carried in a pocket or other receptacle offering restrictive space. Alternatively, retractable antennas are convenient for this purpose because they can be folded away when the radio is not in use. When a large retractable antenna is required, a telescopic tube type antenna may be employed.
A known portable radio is disclosed in U.S. Pat. No. 3,087,117 having two antennas. A first antenna is an internal element type antenna and the second is a retractable element type antenna. In addition, the equipment also contains means for automatically switching between the two elements according to the physical position of the retractable element. Hence, the retractable antenna is operable in an extended position while the internal antenna element becomes operable when the retractable element is in its retracted position.
With dual antenna systems, it is important that both antennas should provide efficient operation under their respective operating condition. For example, while an elongate external antenna may provide better sensitivity and range during normal use, the less efficient internal antenna, usually of the helical type, must provide satisfactory performance during stand-by operation.
U.S. Pat. No. 4,868,576 discloses an antenna for a portable cellular telephone comprising a helical coil at the base of a retractable elongate element. The retractable element, which extends through the helical coil, has non-conductive portions at its two ends, whereby, in the extended position, the elongate element is capacitively coupled to the helical coil, while in its retracted position, the elongate element is substantially de-coupled from the helical coil. In order to improve the performance of the helical coil, said coil is permanently mounted onto the housing of the radio transceiver.
A problem with the arrangement shown in U.S. Pat. No. 4,868,576 is that the elongate antenna and the helical antenna must be designed so as to interact correctly, given that, due to the capacitive coupling, the helical antenna is still effectively in-circuit when the elongate antenna is in its extended position.
An alternative arrangement is disclosed in European Patent Publication No. 0 516 490, in which an antenna assembly comprises an elongate ant mounted in a support and movable between a retracted position and an extended position. A helical antenna element is carried at one end of the elongate member, such that, when the elongate member is extended, the helical antenna is effectively taken out of circuit. Thus, this solution overcomes the problem of the helical antenna being in-circuit during operation of the elongate antenna. However, a problem with this arrangement is that the helical antenna must be supported by the elongate antenna when said elongate antenna is extended, thereby increasing the risk of either of said antennas being damaged during operation.
It is an object of the present invention to provide an improved antenna assembly mountable onto a housing for a radio device. In particular, it is an object of the present invention to provide an antenna assembly mountable onto a housing for a radio device which overcomes the problems identified above.